With the rapid development of electrochemical devices, high performance-electrolyte systems are required. Especially, composites of polymers and room-temperature ionic liquids (ILs), referred to as ion gels, have several advantageous features such as high ionic conductivity, non-volatility, good electrochemical stability, and high thermal stability. In contrast to liquid electrolytes, ion gels do not leak under mechanical deformation, so they are considered promising electrolytes for future wearable electrochemical devices such as energy storage devices, flexible displays, and deformable electrical skins.
Physical properties of ion gels strongly depend on the structure and conformation of polymer gelators. Thus, their design is crucial for achieving highly conductive materials with sufficient mechanical robustness. Several attempts have been made to improve the physical properties such as mechanical properties and ionic conductivity of gel polymer electrolyte (GPE) systems by adjusting molecular characteristics of constituent materials. However, ion gels still suffer from a significant trade-off between ionic conductivity and mechanical properties.
Among the various applications of GPEs to electrochemical devices, some reasearches have expanded the functionality of ion gels by demonstrating low-voltage, flexible electrochemiluminescent (ECL) devices using ion gels containing redox-active luminophores. Electrochemiluminescence (ECL) is a light emission process through electrochemical redox reactions. For example, an electronically excited state of luminophores is generated by an electron transfer reaction between reduced and oxidized species of the luminophores, which corresponds to the annihilation path.
In this thesis, I focused on trade-off problems between ionic conductivity and mechanical property of ion gels for applications to durable electrochemical devices. For this purpose, simple and innovative strategies for achieving non-volatile, durable, freestanding ion gels (FIGs) platforms for ECL devices are proposed. The approach involved molecular design of polymer gelators considering that mechanical characteristics of ion gels strongly depend on those of polymer gelators. I present highly robust ion gels based on a six-arm star-shaped block copolymer of (poly(methyl methacrylate)-b-polystyrene)6 ((MS)6) and an ionic liquid of 1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide ([EMI][TFSI]). Compared to typical ion gels based on linear polystyrene-b-poly(methyl methacrylate)-b-polystyrene (SMS), the (MS)6-based gels show mechanical moduli of more than twice under various strains. Moreover, I propose a star-shaped random copolymer ((polystyrene-ran-poly(methyl methacrylate))6 ((S-r-M)6) as an effective gelator platform to maximize the synergetic effects of the star-shaped molecular architecture and random configuration. Finally, for improving the intrinsic thermomechanical properties of the polymer, poly(methacrylic acid)-ran-poly(methyl methacrylate) (PMAA-r-PMMA)-based ion gel capable of hydrogen bonding with having high glass transition temperature (Tg) is proposed.
In chapter 2, I present highly robust ion gels based on a six-arm star-shaped block copolymer of (MS)6 and an ionic liquid of [EMI][TFSI]. Compared to typical ion gels based on linear SMS, the (MS)6-based gels show mechanical moduli of more than twice under various strains (e.g., stretching, compression, and shear). In addition, the outstanding mechanical property is maintained even up to 180 °C without a gel?sol transition. To demonstrate that (MS)6-based ion gels can serve as effective gel electrolytes for electrochemical applications, tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)32+), a representative ECL luminophore, is incorporated into the gels. In particular, flexible ECL devices based on (MS)6 gels exhibit high durability against bending deformation compared todevices with gels based on linear SMS having a similar molecular weight and a composition.
In chapter 3, I synthesized 6-arm star-shaped random copolymer ((S-r-M)6) for mechanically robust and thermally stable ion gels containing an ionic liquid of [EMI][TFSI]. The (S-r-M)6-based gels exhibited higher elastic modulus (E ~ 1.67 × 105 Pa), which is more than five-times that (~0.29 × 105 Pa) of linear PS-r-PMMA-based ion gels at the same Sty content (~29 mol%), irrespective of applied mechanical strains (stretching and compression). In addition, they showed outstanding thermal stability. For example, the gel-sol transition temperature (Tgel) of (S-r-M)6 gels was ~72 °C, compared with that (~56 °C) of linear PS-r-PMMA-based ion gels. These physical properties of gels were further improved by increasing total molecular weight and the fraction of styrene, giving E of ~3.8 × 105 Pa and Tgel of ~163 °C. The resulting gels were functionalized by introducing electrochemiluminescence luminophores (Ru(bpy)32+). By utilizing the mechanical robustness of the (S-r-M)6 gels, I fabricated emissive electrochemical displays through ‘cut-and-stick’ process. Moreover, the thermally stable (S-r-M)6 gels indicated good dimensional stability, offering a chance to demonstrate ECL devices that operate even at high temperatures.
In chapter 4, I proposed a novel molecular design of polymer gelators. The strategy is that copolymerization of PMMA with IL-insoluble high Tg polymers (PMAA) that can be physically cross-linked by hydrogen bonds. Highly robust (E~1.12 × 106 Pa) and thermally stable (Tgel ~ 202.1 ℃) gels are obtained by judiciously adjusting the molecular characteristics of polymer gelators and gel composition. In the point of the thermal stability issue of the power sources in electric vehicles, military weapons, space equipment, supercapacitors with high energy and power densities that can withstand harsh temperature environments are extremely desirable. Therefore, in the future works, I will investigate the extension of functionality of the PMAA-r-PMMA-based ion gels to the thermal stable and improved dimensionally stable freestanding ion gels (FIGs)-based microsupercapacitors (MSCs).